Communication through the use of modulated digital signals continues to find new and wider application. For example, explosive growth in the use of mobile radio telephones has lead to the adoption of time division multiple access (TDMA) cellular signalling standards, which allow several users to share one radio frequency carrier signal. A transmitted TDMA cellular signal typically consists of an audio signal which has been converted to discrete-time, or digital, symbols. The digital symbols are compressed in time and then typically phase- or amplitude-encoded by quarternary phase-shift keying (QPSK) or quadrature amplitude modulation (QAM). The encoded digital signal is then radio-frequency modulated and is often transmitted in bursts during time slots which have been pre-allocated to each transmitter.
Carrier frequency recovery and symbol timing synchronization and are two of the most critical functions performed by any digital signal receiver In order to properly demodulate any incoming radio frequency signal, the phase and frequency of the carrier signal must be accurately tracked by a local oscillator. In addition, if the incoming signal is digitally modulated, a sampling clock in the receiver should be adjusted continuously in order to maximize the probability of correctly detecting the digital symbols. Accurate recovery of symbol timing information can be particularly difficult in certain applications, such as TDMA cellular, where the radio frequency signal has been subjected to sharp roll-off filtering in order to minimize spectral overlap in adjacent frequency bands.
Most present-day receivers use continuous-time methods for carrier recovery and symbol synchronization. For example, carrier frequency information is typically recovered through the use of a phase-locked loop, and symbol timing information is derived by band-pass filtering the demodulated signal, or other analog techniques.
Certain discrete-time correction techniques have recently been proposed by others. See, for example, U.S. Pat. No. 4,977,580 issued to McNichol, and assigned to Northern Telecom Limited. The technique shown in that patent is to estimate a symbol timing error by interpolating successive samples of the incoming signal. A carrier phase error is then estimated from the symbol phase error. The sampling rate required for such techniques to adequately operate is typically much greater than the symbol rate.
However, these known techniques have been found to be less than satisfactory in certain environments, such as large cities, where TDMA signals are quite susceptible to fast multipath fading.
Other principles of communication theory are of interest as well. In particular, the article by Gardner, William, A., entitled "Exploitation of Spectral Redundancy in Cyclostationary Signals", IEEE Siqnal Processing Magazine, April 1991, pp. 14-36, explains that most signal processing detection methods treat the incoming signal as though it were stationary, in which case the underlying parameters of the physical mechanism which generated the signal do not vary with time. However, as the article points out, most man-made signals, such as periodic modulated carrier signals, are cyclostationary, in that they exhibit second-order periodicities. These second-order properties can be determined by the use of so-called cyclic correlation, which introduces a cyclic weighting factor in a conventional correlation calculation. The Gardner article also points out that certain types of digitally modulated signals, such as the QPSK and QAM signals used in TDMA systems, exhibit second-order periodicity when they undergo a non-linear transformation, like a signal squarer. However, that article does not explain how these periodicities can be exploited to perform carrier recovery and symbol synchronization efficiently.
What is needed is a discrete-time technique for carrier frequency recovery and symbol timing which can adequately tolerate fading and other channel impairments. The technique should be capable of correcting frequency offset and sampling time in as short a time as possible, so that it may be used in certain types of digital signal receivers, such as those used in TDMA cellular systems, which may need to reacquire synchronization at the beginning of each burst. The sampling rate of the carrier recovery and symbol timing technique should also be as small as possible, to minimize the cost and complexity of the receiver.